Generally, heterogeneous metabolic disorders are termed diabetes mellitus with chronic hyperglycaemia as its main finding. It can be due to a disturbed insulin secretion or a disturbed insulin effect or usually both (Petersmann et al., 2019). In 2019, the global diabetes prevalence is estimated at 9.3% (that is, about 463 million people), rising to 10.2% by 2030 and 10.9% by 2045. Additionally, prevalence in urban areas is higher (10.8%) than that in rural areas (7.2%), and that of high-income countries (10.4%) exceeds low-income countries(4.0%) (Saeedi et al., 2019). Type 2 diabetes represents 87 to 91% of the global burden of diabetes (Holman et al., 2015; Koye et al., 2018).

Three out of 4 of people with diabetes live in low- and middle-income countries. However, the lowest world-standardised prevalence of diabetes in adults was in the Africa Region: 3.8% (Saeedi et al., 2019). Prevention and management of diabetes are of paramount importance. The most essential part of diabetes management includes improving the living standard (diet, exercise, weight reduction, smoking) (Khursheed et al., 2019). The pharmacological management includes oral antihyperglycemics and three injectables which are glucagon-like peptide-1 (GLP-1) (as exenatide), pramlintide and insulin. Currently available oral antihyperglycemics are: biguanides, sulfonylureas (SUs), meglitinides, dipeptidyl peptidase 4 (DPP-4) inhibitors, GLP-1 analogues, thiazolidinedione (TZD), alpha-glucosidase inhibitors and sodium glucose cotransporter inhibitors (SGLT2) (Scheen, 2017; Waring, 2016). In Africa, in addition to the conventional treatments, traditional medicine, which is used by 80% of the populations (Bodeker et al., 2005) offers phyto-treatments for the management of diabetes. These plants are widely used because their low cost, perception of their minimal side-effects, availability and knowledge about their use in the treatment of diseases. Many African plants have been shown to have anti-diabetic effects in animal models (Oguntibeju, 2019). Among which, Sclerocarya birrea has shown promising potential. S. birrea (A. Rich.) Hochst. (Anacardiaceae), known as marula in English, is a common tree species in the semi-arid, deciduous savannas of sub-Saharan Africa (Viljoen, Kamatou, and Ba?er 2008; Shackleton et al. 2002); from The Gambia to Ethiopia and Sudan in East Africa (Dimo et al., 2007). S. birrea (A. Rich.) Hochst. subsp. birrea is an indigenous fruit tree very widespread in Burkina Faso (Bationo-Kando et al., 2016, 2009). This review reports the connection between experimental pharmacological properties/bioactive compounds and antidiabetic ethnomedicinal uses. Furthermore, as diabetes is a chronic condition, often requiring life time treatment, the toxicological effects were also discussed. 

A number of scientific search engines were consulted, including PubMed, Google Scholar, ScienceDirect, Scopus, Scifinder, SpringerLink, Web of Science and Wiley Online. Also, only experimental and analytical studies that investigated the antidiabetic effects of S. birrea and/or toxicological effects were included. Review articles, ethnopharmacology and ethnobotany studies were excluded. In this review, data obtained were summarized based on each field, grouped according to themes and arranged in tabular forms for their evaluation.

Pharmacology of S. birrea antidiabetic effects

Twelve studies on antidiabetic effect of S. birrea were retrieved in PubMed and Google Scholar, none of them were performed in humans. All these studies demonstrated antidiabetic effects. The extraction methods were methanolic, ethanolic, methylene chloride, hexane, acetone and aqueous extraction, the latter being the most frequent. All studies used stem bark extract and one study additionally used root extract. The results are summarized in Table 1.

α-amylase and α-glucosidase inhibition

α-Amylase and α-glucosidase are the key enzymes involved in digestion of dietary carbohydrates. The three key products responsible for α-amylase digestion comprise maltose, maltotriose and α-dextrins. After this, an advanced stage of digestion is observed in the small intestine by α-glucosidase which is a class of brush-border bound enzymes that hydrolyses the terminal α-1, 4-linked glucose residues (Koh et al., 2010; Kim et al., 2016; Hanhineva et al., 2010). By means of transporters, mainly sodium-dependent glucose transporter (SGLT1) that can be found in the brush border membrane at the apical side of enterocytes, cells absorb glucose (Röder et al., 2014). Thus, α-glucosidase and α-amylase inhibition reduces blood glucose levels.

Tree studies demonstrated that S. birrea extract inhibits α-amylase and α-glucosidase activities, thus reducing glucose uptake from gut (Mousinho et al., 2013; Mogale et al., 2011; Nkobole et al., 2011). The aqueous and methanol extracts inhibited α-glucosidase and α-amylase activities in a concentration-dependent manner, producing comparable results with acarbose (the positive control) (Mousinho et al., 2013). In vitro bioassay results of the α-glucosidase and α-amylase inhibitory activities of the plant extracts demonstrated strong α-glucosidase and α-amylase inhibition at 97.4 ± 0.04% and 94.9 ± 0.01% (Nkobole et al., 2011). Mogale et al. (2011)investigated the in vitro inhibitory effects of crude S. birrea stem bark extracts against human urinary α-amylase and Bacillus steatothermophilus α-glucosidase and found evidence that human urinary α-amylase (> 50%) were more potently inhibited by acetone and methanolic extracts than acarbose (p < 0.05). The hexane extract proved to be a weaker inhibitor of α-amylase and a strong inhibitor of α-glucosidase (Mogale et al., 2011). While crude S. birrea methanolic extract competitively inhibited α-amylase, there was non-competitive inhibition of α-glucosidase by hexane extracts. All of these lend credence to the idea that polar S. birrea metabolites (likely pseudosaccharides) mostly inhibits α-amylase whereas non-polar S. birrea metabolites mostly inhibits α-glucosidase (Mogale et al., 2011). In addition, plant polyphenols, which is highly present in S. birrea extracts, have been presented as inhibitors of α-glucosidase and α-amylase activities(Havsteen, 1983; McDougall et al., 2005; Matsui et al.,  2007).

Insulinemia increase

Insulin was discovered in Toronto in 1921 by Fredrick Banting and Charles Best (Karamitsos, 2011). Insulin is the dominant anabolic hormone (that promotes dietary carbon source deposition), and its synthesis, quality control, delivery, and action are regulated wonderfully in various organs or “stations” of its bodily journey. Type 2 diabetes is characterized by insulin resistance and insufficient insulin secretion, and type 1 diabetes mellitus, characterized by absolute insulin deficiency (Tokarz et al., 2018; Ruegsegger et al., 2018).

Three studies showed that S. birrea increase plasma insulin level as well as its sensitivity both in vivo and in vitro (Dimo et al., 2007; Ndifossap et al., 2010; Ngueguim et al., 2016). In Ngueguim et al. (2016) study, the plant extract administered in association with supplements in oxidized palm oil and sucrose diet, a significantly (p<0.001) increased the insulin sensitivity index by 264.04 and 417.31%, respectively at 150 and 300 mg/kg doses while 10 mg/kg dose of glibenclamide prompted a reduction of insulin constant value by 355.86% when compared with rats receiving hypercaloric diet. They also found that the plant extracts administration with supplements in oxidized palm oil and sucrose diet provoked hyperglycaemia inhibition induced by glucose 1 h after ingestion. The inhibition at the 150 and 300 mg/kg doses was 31.73 and 36.35% (p<0.001) respectively compared to the control. In a similar vein, glibenclamide induced hyperglycaemia inhibition by 38.88%. Nevertheless, when plant extract was administered, food intake significantly decreased at 150 and 300 mg/kg doses by 15.95 and 22.29%, respectively whereas 24.55% decrease was provoked by glibenclamide. Finally, the extract of S. birrea induced a reduction of glycaemia at a comparable level compared with glibenclamide in diabetes induced rats (Ngueguim et al., 2016).

Dimo et al. (2007) showed that in diabetes induced rats, at the end of 10 h, the plant extract at 150 and 300 mg/kg doses maximally reduced blood glucose concentrations to about 65.8 and 67.0%, respectively, as compared with the basal level. Granted, 10 h after it was administrated, metformin (500 mg/kg) was responsible for the most significant decrease (an 82.4% reduction) in basal glycaemia. In post-prandial hyperglycaemic test, S. birrea (300 mg/kg), like metformin (500 mg/kg), significantly reduced blood glucose concentrations 30 and 60 min, respectively after administration, compared to the diabetic control rats (Dimo et al., 2007). Our team obtained similar results. The aqueous ethanolic extract of S. birrea leaves at the dose of 100 mg/kg body weight significantly inhibited the hyperglycaemia peak (p < 0.001) by 36% in an oral glucose tolerance test with 20% glucose solution in mice fasted for 14 h. However, aqueous ethanolic extracts of S. birrea leaves administered  to   normoglycemic  mice  reduced  plasma glucose by only 0.54%. This suggests a lower risk of hypoglycaemia with either extract in our conditions of use (Youl et al., 2020). In vivo studies showed similar results. Following 24 h of treatment at 5 mg/ml, in insulin-secreting INS-1E cells and isolated rat islets, glucose-stimulated insulin secretion was markedly potentiated by the extract (Ndifossap et al., 2010). Similarly, Youl et al. (2013) found that pre-treatment with S. birrea ethanol (80%) extract reduce considerably the insulin secretion alteration induces by  hydrogen peroxide (H₂O₂) p< 0.05). However, two studies showed no effects on plasma insulin level. Evaluation of insulin secretion in RIN-m5F rat pancreatic β-cells showed no alteration (Mousinho et al., 2013). Gondewe et al. (Gondwe et al., 2008)reached to the same results in an in vitro evaluation. In non-diabetic rats, plasma insulin secretion was not affected by S. birrea extract.

Increase of glucose uptake

The fuel required by most mammalian cells for energy metabolism is supplied by means of glucose transport. Glucose is a very common metabolic substrate that is used both as a fuel and as a signalling molecule (McCall, 2019). Different factors comprising those associated with several aspects of cellular stress regulates glucose transport (McCall, 2019). Glucose uptake depends on a family of eight developmentally regulated glucose transporters, with each having a specific tissue distribution. GLUT 1 has a high affinity for glucose and is insulin independent, being responsible for basal glucose uptake (Beardsall and Ogilvy-Stuart, 2020). One characteristic of diabetes mellitus is glucose homeostasis disruption. Upon food intake, there is increase in blood glucose levels as well as secretion of insulin from pancreatic β cells. As insulin is received, it prompts activation of the insulin signalling pathway in cells possessing insulin receptors, leading to translocation of a glucose transporter (GLUT4), to the plasma membrane, thus expediting glucose influx (Pao et al., 1998; Huang and Czech, 2007; Yoshida et al., 2012). In type 1 diabetes, secretion of insulin is defective, while in type 2 diabetes, cells are insulin resistant (insensitive to insulin), which is likely due to lifestyle. In both cases, cells find it difficult to take up glucose from blood, and as a result, hyperglycaemic situations build up, leading to induction of several diabetic complications (Pao et al., 1998; Huang and Czech, 2007; Yoshida et al., 2012).

Two studies showed that S. birrea extracts increase glucose uptake in cells (Mousinho et al., 2013; van de Venter et al., 2008). Venter et al. (2008)measured glucose uptake in Murine C2C12 myoblasts and 3T3-L1 preadipocytes as well as human Chang liver cells. They found that S. birrea root, stem and bark induce the uptake of glucose, with the highest performance achieved with the  latter.  Mousinho  et  al.  (Mousinho  et al., 2013)evaluated glucose uptake in C₂C₁₂ myotubes, 3T3-L1 adipocytes and HepG2hepatocarcinoma cells and established that it was significantly (p < 0.05) above the positive control (insulin).

Increase of glycogenesis

Glycogenesis is the pathway by which glycogen is synthesized from glucose for storage. Glycogenolysis is a process that deals with glycogen degradation to be utilized as an energy source primarily in liver and skeletal muscle (Patino and Mohiuddin, 2020; Noguchi et al., 2013). Glycogen, which is the main storage form of glucose and prime source of non-oxidative glucose for liver and skeletal muscle, confers significant contributions via its degradation by sustaining normal blood glucose levels and delivering the fuel necessary for muscle contraction (Patino and Mohiuddin 2020). These processes are important to carefully maintain blood glucose levels (Nordlie et al., 1999). Glycogenesis reduces blood glucose levels.

One study showed that S. birrea extract promotes glycogenesis in liver. Gondwe et al. measured glycogen in liver tissue of diabetic and non-diabetic rats. S. birrea extract at 120 mg/kg significantly increases glycogen level after 5 weeks of administration, as much as metformin 400 mg/100 g of protein, compared with control (110 mg/100 g) and glibenclamide (110 mg/100 g). However, the extract had no significant effect on glycogen levels in non-diabetic rats (850 mg/100 g) compared with (800 mg/100 g), metformin (1100 mg/100 g), glibenclamide (1200 mg/100 mg) (Gondwe et al., 2008).

Chemistry analysis

The Bioinformatics and High Performance Computing Research Group of Universidad Católica de Murcia has recently developed a web server, called DIA-DB, for predicting diabetes drugs that employs two different and complementary approaches: a) comparison by shape similarity against curated database of approved anti-diabetic drugs and experimental small molecules, and b) inverse virtual screening of the input molecules chosen by the users against a set of therapeutic protein targets identified as key elements in diabetes. The protein targets are identified by the DIA-DB for the flavan-3-ols. The docking- and similarity-based outcomes proved that S. birrea stem-bark extracts anti-diabetic potential mayresult from the collective action of multiple bioactive compounds regulating and restoring several dysregulated interconnected diabetic biological processes (Pérez-Sánchez et al., 2020). The observed reduction in hyperglycaemia and improvement in the oral glucose tolerance  test   is   possibly   attributed  to  GCK,  AMPK, MGAM and AMY2A regulation by the S. birrea compounds. AMY2A and MGAM inhibition slows down digestion of carbohydrate and thus lowers the postprandial blood glucose level. Activation of GCK will also lead to a reduction in serum glucose levels by promoting glycogenesis and glycolysis through the phosphorylation of glucose to glucose-6-phosphate (Pérez-Sánchez et al., 2020; Xu et al., 2017). AMPK activation is connected to increased uptake of glucose by the muscles as well as decreased production of glucose by the liver leading to a reduction in blood glucose levels (Pérez-Sánchez et al., 2020; Yan et al.,2013). The observed in vitro stimulation of glucose uptake by liver, muscle and adipose cells by S. birrea extracts may thus be due to AMPK activation by compound catechin, epigallocatechin, epicatechin, citric acid and gallic acid (van de Venter et al., 2008; Pérez-Sánchez et al., 2020). The incretin hormones half-life will be boosted by DPP4 inhibition thus improving secretion of insulin and allowing time for blood glucose levels to normalize. In a similar vein, potentially-inhibiting HSD11B1 compounds can inhibit production of glucose by the liver and enhance the sensitivity of glucose-dependent insulin (Pérez-Sánchez et al., 2020; Abbas et al., 2019). By inhibiting these receptors, S. birrea increase insulin secretion thus reducing blood glucose (Pérez-Sánchez et al., 2020).

Other beneficial effects in diabetes

Antioxidant activity

Under conditions of sustained hyperglycaemia, the oxidative stress is greatly increased.  Increased generation of free radical, emanating from glucose auto-oxidation, improved glycation and polyol pathway activity alterations, exerts a modular effect on the oxidative stress level (Araki and Nishikawa, 2010). These free radicals can cause damage to β-cells (Kokil et al., 2010)and the oxidation homeostasis imbalance will develop insulin resistance which is a risk factor for diabetes type 2  (Asmat et al., 2016). Thus, it might be necessary to consider antioxidant therapies targeting precisely the diabetes-induced oxidative stress mechanisms as part of the therapeutic approach towards preventing downstream diabetic complications (Araki and Nishikawa, 2010).

Mousinho et al. (2013) demonstrated that S. birrea extract has antioxidant activity. Both the ABTS•+ and DPPH radicals were scavenged by the extracts in a concentration-dependent manner. The S. birrea methanol extract gave the lowest IC₅₀ value (2.2 µg/ml), suggesting that it has the strongest radical scavenging activity. In both assays, the extract was found to be more potent than Trolox. Armentano et al. (2015) also investigated S. birrea methanolic root extract antioxidant activities. Antioxidant activity was measured by 4 different tests (ABTS,   nitric  oxide (NO), superoxide anion (SO), and β-carotene bleaching (BCB) assays) and in each one it is demonstrated to be dose-dependent  (Armentano et al., 2015). The leaves extract has also shown antioxidant proprieties (Braca et al., 2003). Three compounds (-)-epicatechin 3-O-galloyl ester (10) and (-)-epigallocatechin 3-O-galloyl ester (11) have a Trolox equivalent antioxidant capacity (TEAC) value (∼3 mM) higher than that of the reference compound quercetin. Tawi et al. in 2016 showed that water and methanol extracts of bark of S. birrea and exhibited excellent antioxidant activities with their 50% inhibitory concentration of DPPH radical ranging from 0.28 - 0.02 to 0.40 - 0.02 µg/ml. This is quite impressive when compared to the positive control vitamin C, which had a 50% inhibitory concentration of 10.62 -0.87 µg/mL (Tawi et al., 2016).

Youl et al. (2013) also demonstrated that the S. birrea protected ß cell viability and functionality against oxidative stress exogenously induced by hydrogen peroxide (H₂O₂). After 2 h, S. birrea (10 µg/ml) partially deterred alteration of 50 µM H₂O₂-induced viability. The functionality alteration (insulin secretion) is totally prevented by 1 µg/ml of S. birrea after two hours (Youl et al., 2013).

Lipid and cholesterol

People with diabetes mellitus are at an increased risk for cardiovascular diseases, they have more than a 2-fold increased risk of cardiovascular death compared with persons without diabetes. Cardiovascular death accounts for more than 75% of all deaths among persons with diabetes mellitus (Selvin et al., 2004; Khan et al., 2008). Patients with type 2 diabetes often exhibit an atherogenic lipid profile (high triglyceride and low HDL cholesterol) which greatly increases their risk of cardiovascular diseases compared with people without diabetes (Selvin et al., 2004; Windler, 2005). Interestingly, attempts to reduce cardiovascular risks resulted in the improvement of HbA1c even in the absence of any specific intervention targeted at improving glycemic control (Giansanti et al., 1999). Some investigators reported significant correlations between HbA1c and lipid profiles and suggested the importance of good management of diabetes in controlling dyslipidemia (Khan et al., 2008; Ko et al., 1998).

Clementine et al. (2018) evaluated the effect of ethanolic extract of S. birrea trunk bark. They found that the triglyceride level in the treated rats decreases. This decrease is very significant (p <0.01), at all doses tested (100, 200 and 300 mg / kg). Similarly, the total cholesterol level decreases. This decrease is highly significant (p <0.001) in all treated rats, regardless of the dose. The analysis shows that the HDL cholesterol level decreases very significantly (p <0.01) at a dose of 100 and 200 mg / kg. This decrease is highly significant (p <0.001)  at   a  dose  of  300  mg / kg  (Clémentine  et  al. 2018). Borochov-Neori et al. (2008) demonstrated that the fruit of S.birrea also lowers blood lips levels. Three-week administration of the juice as a food supplement to healthy subjects significantly reduced their serum total cholesterol (by 8%), LDL-cholesterol concentration (by 17%), and triglyceride level (by 7%), increased their serum HDL-cholesterol level (by 10%), and attenuated serum oxidative stress (Borochov-Neori et al., 2008).

Toxicology of S. birrea

Nine studies on toxicological effects of S. birrea were retrieved in PubMed and Google Scholar, none of them were performed in humans. These studies used aqueous and organic extracts of leave, stem bark, root, kernel and fruits. In vivo acute and subchronic studies were performed in rodents and in vitro tests in different study-models. Overall, the studies indicated little toxicity of S. birrea. The results are summarized in Table 2.

In vivo toxicological investigation of S. birrea

Five studies evaluated acute and subchronic toxicity of S. birrea in rodent.

Acute toxicity

No study reported acute toxicity, the LD50 being greater than 500 mg/kg of body weight (Lorke, 1983). Muhammad et al. (2014) reported that LD 50 was greater than 3000 mg/kg body weight in both kernel and fruit peel (Muhammad et al. 2011); Mawoza et al. (2016) estimated that LD50 was greater than 2000 mg/kg (Mawoza, Tagwireyi, and Nhachi 2016); while Baba et al. (2014) reported that LD 50 were 566 and 800 mg/kg body weight for Aqueous and ethanolic extracts of the plant leaves (intra-peritoneal), respectively (Baba et al., 2014). However, some signs appeared at high doses. Behavioural changes in the form of reduced mobility were however noted in the animals given ≥1000 mg/kg of S. birrea stem bark aqueous extract (Mawoza et al., 2016).

The animals after 24 h of the administration of the aqueous and ethanolic leaves extracts of S. birrea show weight loss, increased body temperature and heartbeat and a slight decrease in food consumption (Baba et al., 2014).

Subchronic toxicity

Subchronic toxicity evaluation of S. birrea extracts revealed some signs of toxicity.  Clementine et al. (2018) found that the extract increased significantly, the urea creatinine,  AST,  ALT  and  uric acid suggesting liver and kidneys toxicity at dose of 100, 200 and 300 mg/kg trunk bark ethanolic extract (Clémentine et al. 2018). Mawoza et al. shown that animals in the 1000 and 2000 mg/kg groups showed a significantly (p<0.05) smaller growth rate. This was associated with increases in direct bilirubin, total protein, albumin, AST and ALT supported by histopathological changes to the liver and kidneys. Same results were reported by Muhammad et al.: some subchronic toxicity signs at doses of 4000 mg/kg of peel and kernel extracts. This included reduction in the body weights with an increase in serum total protein, albumin, bilirubin, transaminases, creatinine, urea, uric acid and electrolytes, suggesting liver and kidney toxicity  (Muhammad et al., 2011, 2014). However, no subchronic toxicity at doses lower than 2000 mg/kg; which is even higher than then clinical doses.

In the contrary Gondwe et al. described that S. birrea stem-bark ethanolic extract has reno- and cardio-protective effects in diabetes mellitus. Daily S. birrea treatment (120 mg/kg) for 5 weeks did not significantly affect renal fluid or electrolyte handling in non-diabetic and STZ-induced diabetic rats throughout the experimental period. Chronic S. birrea treatment, however, significantly (p< 0.01) decreased plasma urea and creatinine concentrations of STZ-diabetic rats with concomitant increase in glomerular filtration rate by comparison with control rats at the corresponding period (0.7 ±0.2 vs. 1.4 ±0.3 ml/min) (Gondwe et al., 2008).

In vitro toxicological investigation of S. birrea

The literature has shown contradictory subchronic toxic effects of S. birrea extracts. Some studies concluded to reduction in cell viability while others concluded in non-cytotoxicity. Gondwe et al. (2008) investigated the exposure of kidney cell lines of the proximal (LLCPK1) and distal tubules (MDBK) S. birrea extract on cell viability. In both cell lines, there was a dose-dependent decrease in cell viability, with significance at the higher concentrations and 48 and 72 h treatments. The LLCPK1 cells were more sensitive to S. birrea extract treatment at higher concentrations (600-1000 mg/ml) than the MDBK cells. But, as mentioned by authors, the high doses (600-1000 mg/ml), however, may not be equivalent to in vivo doses. In Venter et al. study on Chang liver, 3T3-L1 adipose and C2C12 muscle cells, the organic extracts were found to reduce viability by around 10%, while aqueous extracts had negligeable effects on viability. The root aqueous extracts even increased cell viability by around 5%.

Mousinho et al. (2013) found no cytotoxicity on cell lines tested; 3T3-L1, C2C12, HepG2 and RIN-m5F with IC50 values > 100 µg/mL (Mousinho et al., 2013). Ndifossap et al. (2010) obtained similar results. This revealed that concentrations up to 10?μg/ml S. birrea extract did not induce toxic effects and preserved differentiation of  INS-1E  cells  upon  S.  birrea  exposure (Ndifossap et al., 2010). No data on chronic toxicity were found to support the safety of long terms usage for diabetes management.

Extracts of S. birrea show antidiabetic effects in cellular and animal models. These effects are due to the increase of insulin secretion, increase glucose uptake and increase of glycogen secretion. The extracts have also benefits effects in diabetes which include antioxidant activities and reduction of cholesterol and lipid blood levels. These antidiabetic effects are associated with good safety profile. The extract did not exhibit acute nor subchronic toxicity. However, diabetes being a chronic condition, one must be cautious with regard to the lack of data on chronic toxicological effects. In spite of the promising data on antidiabetic effects, today, no study has investigated its effects in humans (Table 1).

The authors have not declared any conflict of interests.

The authors appreciate the African Science Partnership for Intervention Research Excellence (Afrique One-ASPIRE) for training and technical support. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.